BRASS 12 gauge dummy rounds / snap-caps / fake bullets. Looks like the ones used by the US Army during WWI and World War Two in the Model 1897 Trenchguns.
A-Zoom Action Proving Rimfire Dummy Rounds - .22 LR, 6pk [12208]
12 Gauge Snapcaps
Inert Dummy Shotgun Shells & Cartridges
Inert Dummy Shotgun Shells & Cartridges
Advanced Tactical 8x57 Mauser Snap Caps 8mm Dummy Rounds (5 Pack)
12 Gauge Brass - Snap Caps Dummy Rounds
All American Training Rounds - Dummy 12 Gauge 2 3/4
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B's Dry Fire Snap Caps - 12 Gauge
MultiBrief: Why every owner of a break-open shotgun should know